The subject matter of this invention is related to a static, current-limiting, bilateral force commutated switch with a reverse parallel pair of thyristor stacks as the main conduction element.
It has been long known that current limiting in an electrical circuit subject to high values of overload current is desirable. In the fuse art, for example, current limiting fuses are known which not only melt upon the presence of an overload but operate to reduce the amount of overload current during the short interval between the time that circuit interruption begins and the time that circuit interruption is completed. It has been long recognized that in the latter time interval though of relatively short duration, perhaps only a few hundred microseconds, there is sufficient time for the current in the circuit to rise to such a high value as to permanently damage those elements for which the circuit protective device was designed to protect. Analogously, certain current limiting apparatus is known in electrical circuits protected by circuit breakers, non-current limiting fuses or the like. Take the case of mechanical circuit breaker apparatus, for example. Generally these devices are highly reliable and well thought of in the art of circuit interruption. However, since they are mechanical devices, interruption time may become relatively long. As was mentioned previously, the value to which a fault current may rise in these relatively long circuit opening times can become prohibitive in terms of protection for the circuit. To remedy this the concept of utilizing a force commutated circuit in series with the mechanical circuit breaker was introduced into the art. The circuit essentially consists of oppositely disposed, parallel connected, unidirectional, solid state devices such as thyristors. Connected across the oppositely disposed thyristors is a relatively high resistive element for current limiting and a commutating circuit. In operation, the control circuit for the gates of the thyristors maintains the thyristors in a conductive state for alternating current during a normal operating situation. Consequently, the normal operating alternating current sees the relatively small resistance or forward voltage drop of a thyristor during each half cycle. Even when peak inverse voltage constraints in the circuit require that numerous thyristors be connected in series stacks, the net resistive effect and voltage drop effect though somewhat undesirable is usually deemed acceptable in view of the very desirous current limiting operation provided by the commutating circuit and resistive means which parallels the normally conducting thyristors. In the event of the onset of a fault current or the like, appropriate sensing apparatus associated with the commutating circuit quickly determines that a fault current of unacceptable magnitude is in the process of developing. The sensing circuit then quickly reacts to this phenomenon by deenergizing the gates of the thyristors, thus attempting to render the thyristor non-conductive. Furthermore, the sensing circuit switches the commutating circuit into operation. The commutating circuit usually consists of a precharged capacitor which sinks current away from the conducting thyristor and through the capacitor. It does this very quickly, often within a time span of a few microseconds. The reason for this is well known in the art. A thyristor will not stop conducting merely because its gate signal has been removed. Two additional characteristics must be satisfied before the thyristor will cease to conduct. First, the current flow in the thyristor must be reduced to zero and, second, the anode-to-cathode voltage of the thyristor must be reverse biased for a relatively short period of time (recovery time) to sweep out the carriers that might otherwise lead to reconduction. It can be seen therefore that the capacitor generally provides a dual function; it quickly sinks away the current from the main thyristor path thus reducing the current in that path to zero to meet the first characteristic, and it reverse biases the thyristor thus meeting the second requirement. In performing its second function, i.e. reverse biasing, the voltage of the capacitor is usually so high that its reverse biasing characteristic is known as "hard" reverse biasing. This is usually associated with high voltage but, more importantly, it is associated with utilizing the capacitor as a voltage source rather than a current source. Generally, the means for connecting the charged capacitor to the circuit for accomplishing its stated purposes is an auxiliary switching thyristor. Unfortunately the gating of the auxiliary switching thyristor may cause current to be commutated away from the main thyristor and into the capacitor circuit at such a high rate as to destroy the auxiliary thyristor. It is well known that thyristors have a maximum rate of rise of current with respect to time which they can tolerate without being destroyed. In order to solve this problem in the prior art, an inductor is used in series with the capacitor to limit the rate of current rise. When the main thyristor ceases to conduct in the prior art, the hard capacitor voltage appears across that thyristor, as reverse bias, and in series with the system voltage. This results in the first drawback of a hard commutation circuit; namely, since the system voltage is boosted by the capacitor voltage, the fault current rises even faster than before. The second drawback of a hard commutation circuit is that the current limiting resistor which is connected directly across the thyristor starts to deplete the capacitor charge from the moment the compensating branch is actuated. Consequently, a larger capacitor is needed to provide the necessary reverse bias time (recovery time) than would be needed if the resistance were not present. In prior art, U.S. Pat. No. 3,921,038 issued Nov. 18, 1975 to Kernick et al entitled "Static Surge-Current Limiter" and U.S. Pat. No. 3,737,759 issued June 5, 1973 to Pollard entitled "Static Switch Including Surge Suppressing Means", the latter-mentioned problem was circumvented by connecting a current limiting resistive element in series with a diode but in parallel with the capacitor. As a result, the circuit branch containing the resistor does not start to conduct until after the reverse bias interval is over. This saves commutating capacitance at the cost of utilizing a relatively high voltage diode which must have a short duration current rating equal to the limited transient fault current. The third drawback of a hard commutating circuit lies in the fact that the previously discussed inductance is utilized merely to limit the rate of rise of current through the discharge circuit. The presence of the inductance, though needed, actually degrades the circuit to a certain extent because reverse biasing of the thyristor will not start until after the entire line current has been commutated into the capacitor, by which time a non-negligible portion of the charge of the capacitor may have been depleted. In the previously mentioned prior art patents, a number of things were done to eliminate certain problems associated with the previous generation of current limiters. One of the things done was to introduce the concept of the "soft" commutation circuit. With soft commutators, the capacitive element acts not as a voltage source but as a current source. This soft commutating circuit implies that the conducting thyristor, i.e. the one to which the reverse bias must be supplied, is turned off by a very modest reverse bias voltage. This voltage is obtained by a resonant discharge of the precharged capacitor through an inductor and a diode connected in reverse across the conducting main thyristor. The forward voltage drop of the diode, therefore, appears as reverse voltage across the thyristor from the instant the capacitor discharge current rises above the value of the main current until the instant the capacitor discharge current drops below the value of the main current. During this interval, the thyristor is provided with soft reverse bias. It is immediately after this interval, but during the alternating current half cycle of interruption, that the net current which attempts to flow through the thyristor reverses again. That is, the capacitor discharge current falls below the value of the main current. However, with the thyristor now recovered and thus turned off and the diode blocking, the main current divides between the branch containing the resistive element and the commutating branch containing the capacitive element and the inductive element L. This soft commutating circuit removes one of the problems associated with the inductor. In this case, the inductor L no longer degrades the circuit, but is utilized in conjunction with the capacitor to tune the circuit to obtain the main current reversal. Furthermore, the other two drawbacks associated with hard biasing are also eliminated. First, the voltage inserted into the line is soft or of relatively low value. Thus, the circuit does not contribute significantly to the rate of increase of the fault current. Secondly, the resistive element utilized for current limiting, although directly in parallel with the thyristor switch, carries only negligible current. However, in overcoming the aforementioned drawbacks, other drawbacks appeared. One is the need for the utilization of a series of diodes stacked together for the purpose of carrying the discharge current of the capacitive element during the period of time when that current is larger than the main current to provide the previously described soft bias voltage. It is noted that in the prior art, these additional diode stacks are always present in the main current conduction path for the alternating current. Consequently, these stacks must be rated to continuously carry full load current and to support the system voltage plus transient overloads and thus constitute a significant cost item. Furthermore, the conduction losses in the diode may increase the switch losses by approximately 60% which is highly undesirable in high power applications. Therefore, it would be desirable if the utilization of the soft biasing principle but with elimination of the drawbacks associated therewith could be implemented. It would be further advantageous if the solution would utilize an existing circuit element to perform extra circuit functions above and beyond what was described in the prior art, thus providing a significant cost reduction. Furthermore, it would be advantageous if apparatus could be found which utilized the soft biasing technique in which the suppression of transients associated with a switching and commutating operation could be affected. Furthermore, it would be advantageous if a commutating circuit could be found in which repeated reversal of the capacitive discharge current could be permitted or denied as required.